It's the most unlikely suggestion so far (because of linguistic usage), but that doesn't necessarily mean it's wrong. Didn't anybody ever ask Jimmy or Davy what they meant (assuming either of them understood anything in particular by it)?

MacColl (ref above) glossed Mary Brookbank's "ablaw" as "from everywhere", for what that's worth. "Oblaw" (in the uncredited example mentioned by "Fliss" earlier) is probably a mis-hearing of "ablaw" as heard by somebody on a modern recording of her set; since the people who run that "Celtic lyrics" site couldn't be bothered to say where they got it, we can't be absolutely sure.